Can you upgrade from an iPhone 3GS to an iPhone 5s?

I've had an iPhone 3GS for a while now and I'm almost due for an upgrade. Do I have to get an iPhone 4 or can I upgrade to an iPhone 5s? I have AT&T by the way. Can I do it without having to pay a thousand dollars?

No you can go straight to the iPhone 5! I know for verizon the 4 is free, and depending on which model you want here are the costs with the eligible upgrade;

Iphone 5c (Plastic colored ones): $99.00

Iphone 5s (Top Model): $200.00

And the older Iphone 5s models are even cheaper, but I'm not sure of the cost.

You can upgrade to any at&t phone you want with a new two year contract hope this helps.

Yes but if you want to trade it in I think only 4 or later are available on trade in, so you wouldn't need to sell the 3GS on your own.